Under certain conditions, wall-bounded flows separate. At separation, the viscous layer departs or breaks away from the bounding surface. The surface streamline nearest to the wall leaves the body at this point; the rotational flow region next to the wall abruptly thickens. The normal velocity component increases, and the boundary layer separates. Due to large energy losses associated with boundary layer separation, the performance of many practical devices is often controlled by the separation location.
For example, the loss of lift, commonly referred to in the literature as "stall", results from the separation of the boundary layer in the airstream flowing over the wing as a result of an adverse pressure gradient which cannot be negotiated by the boundary layer. Such a separation is induced by an increase in the angle of attack of the wing above a predetermined maximum, and limits the maximum lift of a wing of given dimensions.
A similar separation of the boundary layer occurs in diffusers having an enlargement for slowing the fluid flow and recovering pressure. Thus, the angle of the diffuser surface at the enlargement is limited, typically to a 60-degree equivalent angle, in order to prevent separation of the boundary layer.
On the other hand, if separation is postponed, the pressure drag of a bluff body is decreased, the circulation and hence the lift of an airfoil at high angle of attack is enhanced, and the pressure recovery of a diffuser is improved. Separation control is of immense importance to the performance of air, land or sea vehicle, turbomachines, diffusers and variety of other technologically important systems involving fluid flow and fluid mixing.
Apparatus for delaying the separation of flow from a solid surface (such as a wing) has been disclosed; see, e.g., Israel Wygnanski et al, "Method and Apparatus for Controlling the Mixing of Two Fluids", U.S. Pat. No. 4,257,224, issued Mar. 24, 1981, and Israel Wygnansid "Method and Apparatus for Delaying the Separation of Flow from a Solid Surface", U.S. Pat. No. 5,209,438, issued May 11, 1993. The perturbation-producing elements in both of these patents are of mechanical interference, with the flow field or induced oscillations by periodically injecting a fluid jet via a rotating nozzle. Further, the perturbations induced in the flow are predetermined directed.
Substantial alteration of the flow conditions in a fluid stream may be effected by the application of controlled disturbances to the boundary of or within the stream by one or more piezoelectric actuators; see, e.g., Ari Glezer et al, "Method and Apparatus for Controlled Modification of Fluid Flow", U.S. Pat. No. 5,040,560, issued Aug. 20, 1991, and Michael Amitay et al, "Aerodynamic Flow Control Using Synthetic Jet Technology", 36.sup.th Aerospace Sciences Meeting & Exhibit (Reno, Nev.), Jan. 12-15, 1998 (AIAA 98-0208).
Apparatus for producing either fluid perturbations with zero net mass-flux or simultaneous perturbation and constant flow is needed. Such apparatus would be useful for controlling the separation of a boundary layer of a fluid stream flowing over a solid surface.
It is therefor an object of this invention to provide a new simple and efficient fluid control apparatus. The output of the apparatus is an oscillating air stream derived by a configuration of piezoelectric elements confined in a resonance cavity. The piezoelectric elements can be driven to produce an output jet at a predetermined frequency or an output jet perturbation at different frequencies band sensed by a detector or programmed in advance. The apparatus of the present invention is to be distinguished from the possible perturbation-producing elements described in Wygnanski et al and in Wygnanski, both cited above. The perturbation producing elements in both of those patents are of mechanical interference with the flow field or induced oscillations by periodically injecting a fluid jet via a rotating nozzle. In contrast, the present invention apparatus provides zero net mass flux jets.
Another difference is that the perturbations induced in the flow of the above-mentioned Wygnanski patents are predetermined directed (see also the Glezer et al and Amitay et al references cited above), whereas in the present invention, the direction of the induced perturbation can be electrically controlled without any geometrical or mechanical changes. In addition, no physical change is needed for the jet output nozzle or the shape of the surface on which the flow is being controlled or the jet perturbations apparatus positioned underneath the surface. The perturbations in the above-mentioned patents to Wygnanski et al and Wygnanski are two-dimensional (2-D) without any possibility for longitudinal modulation, nor even staggered activation.
In the present invention, the shape of the 2-D disturbance jet can be easily modified to 3-D, thereby allowing longitudinal modulation in the frequency, amplitude and phase sense. The present invention allows control of unlimited and different phase lag between unlimited and different longitudinal sections. The present invention also allows control of the perturbation frequency as a 2-D perturbation or 3-D for different longitudinal sections. In the present invention, the controlling features, namely, the frequency changing and the longitudinal modulation of phase and amplitude can be easily achieved electrically.
The apparatus of the present invention is also to be distinguished from previously-known techniques, such as those disclosed by Glezer et al and Amitay et al, which aim to create a "synthetic jet" by driving one side of a cavity using a piston or a piezoelectric diaphragm in a periodic manner. Those techniques suffer from poor frequency response band. Eventually, they have only one discrete resonance frequency. Modulation of the input feeding power (pulsed modulation excitation) accomplished by transient response time causes limitation for modulation at higher frequencies, while the modulated input feeding causes a modulated output signal with higher harmonics. In the present invention, there is no need for modulated input while the output is a pure frequency controllable perturbation jet. Another disadvantage of the previous techniques is geometrical length limitation, while the present invention allows infinite length.